This invention relates generally to helium compressor units for use in cryogenic refrigeration systems and, more particularly, to an oil-lubricated helium compressor unit that is fail-safe in that pressurized oil-free helium gas is delivered over the extended life of the unit. Oil-lubricated air conditioning compressors have become standard for delivering pressurized helium to GM type cryogenic refrigerators. The ability to use these relatively inexpensive but reliable compressors results from the development of oil separators and adsorbers that reliably keep oil out of the cold expander of a GM type refrigeration system for periods of several years.
At present, GM refrigerator manufacturers recommend replacing the adsorber at 10,000 to 30,000 hour intervals. This time interval depends on the rate at which oil carries over from an oil separator that receives the high-pressure gas discharge from the oil-lubricated compressor. Oil carryover in the refrigerant gas from the separator goes to an adsorber. The capacity of the adsorber for holding oil, and the degree of risk a user is willing to accept before replacing the adsorber(s) determine the time interval without failure. Carryover of oil from the adsorber would allow oil entrained in the refrigerant gas to carry into the cold end of the system, where the oil adversely affects performance of the GM type expander. It is relatively expensive to clean up the oil once it is in the cold end of the GM refrigeration unit.
In order to avoid the risk of oil carryover, manufacturers and users tend to be conservative by allowing for a good margin of uncertainty in predicting adsorber life. Compressor manufacturers are also conservative in designing the sump of the compressor to hold enough oil so that a considerable amount of oil can be lost from the compressor before the bearings are starved of oil and in danger of seizing.
A data analysis of compressor units manufactured by the assignee of the present invention indicates that such compressor units for cryogenic systems using helium gas typically hold two to three times as much oil as the adsorber can physically retain. Thus, unless there is a program to shut down compressor operation before the adsorber is filled, an inherent danger exists for carryover of oil from the adsorber to the cold end of a connected system. Fluctuations in oil level in the compressor due to changes in ambient temperature, while small, may still require consideration when charging a compressor with oil.
Oil is typically added to a compressor when the adsorber is replaced for the third or fourth time. This oil addition is intended to make up for oil that is removed with the adsorber. However, there is considerable uncertainty in knowing how much oil, if any, to add to the compressor; and sometimes the compressor is overcharged with oil.
Recent improvements in the design of oil separators have resulted in oil carryover rates to the adsorber being less than 20 grams per year for a ten cubic feet per minute helium compressor that draws about 6 kilowatts of power. The sump in this compressor holds about 1500 grams of oil. Therefore, it would take approximately 75 years (1500÷20) to lose enough oil for the bearings to seize. A reasonably sized adsorber can hold about 500 grams of oil so that it is reasonable to recommend that the adsorber be left in place for the normal 10-year life of the system (500÷20=25). For operation beyond ten years, it is generally recommended that the adsorber be replaced; but there is no actual need to add oil when starting with 1500 grams of oil. Clearly everything is overdone, giving a sense that failures (oil carried beyond the adsorber) are avoided.
Having a ten-year service interval based on the adsorber size can reduce ongoing service cost, but does not remove the risk of oil carryover in the event that the oil separator or oil return circuit has a failure. If the adsorber can hold all of the oil that might leave the compressor before the system shuts down, and retain all of the oil when it enters the adsorber at the high rate that might exist when there has been a failure in the oil separator, then the risk of oil carryover from the adsorber is non-existent despite a separator failure. The oil entrainment rate for the conventional compressor, used in the numerical example described above, might be as high as 120 grams per hour. Therefore, the adsorber must be able to collect oil, in that example at this rate (120 grams per hour) without any carryover to the cold end.
What is needed is an oil-lubricated helium compressor unit that operates over an extended life on an oil charge that is sufficient for the desired life but is limited so that the normal carryover does not exceed the capacity of the adsorber and in the event of a failure of the oil separator does not allow oil to carryover to the expander.
Generally speaking, in accordance with the invention, a fail-safe oil-lubricated helium compressor unit is provided having extended life with oil-free delivery of compressed helium.
In the oil-lubricated helium compressor unit in accordance with the invention, the adsorber is sized so that all of the oil that might be transferred from the compressor to the adsorber before the system shuts down can be retained by the adsorber. No oil is ever transferred or transferable out of the unit to, for example, the expander in a GM type refrigeration system. Thus, the compressor itself will shut down because of a protective switch or even seize for lack of oil before any oil carries outside the compressor unit. Components are sized so that under normal circumstances, the unit and the connected refrigeration system can run for more than a selected design life, for example, ten years, before the compressor shuts down because the limit of oil that can be transferred to the adsorber has been reached.
Recognizing that safety factors must be given consideration when sizing and charging components, an ability to run for a predetermined life before the compressor system shuts down can be interpreted that:
(a) The adsorber is sized to retain as much oil as might leave the compressor over the life of the system plus a safety margin of at least approximately 25%.
(b) Should there be a failure of the oil separator or compressor oil return mechanism, then the adsorber retains the oil which enters at the maximum rate that can leave a failed oil separator which may be the same rate that the oil leaves the compressor. In other words, operation is without any carryover from the adsorber, terminating in compressor shut-down.
(c) For an x-year life, the oil separator is efficiently effective so that less than 100/x percent of the oil is transferred from the compressor to the adsorber under normal operation each year. Also, there must be sufficient oil initially that can be transferred from the compressor to the adsorber for x years of operations under those conditions. In other words, for a 10-year life, less than 10% of the oil is xe2x80x9clostxe2x80x9d from the compressor and retained by the adsorber per year.
(d) The adsorber needs no service over an x-year period. Therefore, the separator and adsorber may be combined in a single vessel.
Accordingly, it is an object of the present invention to provide an improved oil lubricated compressor unit with an adsorber capable of holding the entire anticipated net oil output of the compressor during the intended life of the unit.
Another object of the invention is to provide an improved oil-lubricated helium compressor unit having an adsorber capable of absorbing oil at a rate equal or greater than the maximum rate that it might enter the adsorber.
Still another object of the invention is to provide an improved oil-lubricated helium compressor unit that can operate for at least ten years without risk of failure due to oil carryover into an associated refrigeration system.
A further object of the invention is to provide an improved oil-lubricated helium compressor unit with an adsorber that can contain all of the lubricating oil that might be pump-out of the compressor and retained by the adsorber.
Yet another object of the invention is to provide an improved oil-lubricated helium compressor unit that is more economical to produce than prior art units.
Still other objects and advantages of the invention will be apparent from the specification.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements, and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.